Modern magnetic storage devices, including, for example, hard disk drives (HDDs), may use various forms of write heads. FIG. 1 shows a typical inductive write head 100 along with a magnetoresistive read head 105 and a magnetic disk 110. The magnetic disk comprises a recording layer 112 and a magnetically soft underlayer 114. The inductive write head 100 includes two ferrite cores 120. One of the ferrite cores 120 is partially surrounded by a coil 130 to produce an electromagnet. Applying current to the coil 130 generates a strong magnetic field, which forms a fringing field 140 that extends out of the two ferrite cores 120 into the magnetic disk 110 that is moving beneath it. Writing logical data on the magnetic disk 110 involves reversing the polarity of the current through the coil 130 (i.e., the write current) in response to the logical data being recorded to create a pattern of two oppositely-oriented remanent states on the magnetic disk 110 that represent the logical data. The remanent states may be vertically aligned when utilizing perpendicular recording (as shown) or may be horizontally aligned when utilizing longitudinal recording. Under the “non-return-to-zero, inverted” (NRZI) coding protocol, for example, a logical “1” is represented by a change in remanent states from one orientation to the other.
Unfortunately, storing data in this manner is not without issues. When the write current of an inductive write head is switched off, it takes time for the write head to relax back to its net-zero magnetization state. During this relaxation process, residual magnetization from the write head can erase or degrade prewritten data as the head continues to move over the magnetic disk. This failure mode is known as erase-after-write (EAW) or pole tip remanence. When EAW events occur, the effects are often catastrophic. The erased or degraded data may be user data or even the fixed servo sectors that are used by the HDD in determining the radial position of the write head.
One method for mitigating EAW is to apply a degaussing current waveform (DCW) to the write head immediately after performing a write operation. The typical DCW is characterized by a write current that oscillates between opposite polarities at a fixed frequency but with a decreasing amplitude over time. Such a DCW has the effect of switching the write head magnetization polarity in one direction and then in the other at a fixed frequency while gradually decreasing the magnitude of these oscillations so that the write head ultimately ends up in a relaxed, net-zero magnetization state. Nevertheless, such known DCWs are not always effective. There are still many write head designs where conventional DCWs are not entirely effectual in achieving a net-zero magnetization state. EAW incidents therefore persist.